Secretion of bicarbonate provides a mucosal protection at the epithelial surfaces in the gastrointestinal tracts. The mucosa is exposed to noxious agents, including high concentrations of ethanol and medications such as aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs). In each organ, bicarbonate secretion protects potential mucosal damage induced by acid, pepsin, certain drugs such as NSAIDs, and bacterial infection (News Physiol. Sci. 16, 23-28, 2001, the reference is herein incorporated by reference).
Prostaglandins (hereinafter, referred to as PG(s)) are members of class of organic carboxylic acids, which are contained in tissues or organs of human or other mammals, and exhibit a wide range of physiological activity. PGs found in nature (primary PGs) generally have a prostanoic acid skeleton as shown in the formula (A):

On the other hand, some of synthetic analogues of primary PGs have modified skeletons. The primary PGs are classified into PGAs, PGBs, PGCs, PGDs, PGEs, PGFs, PGGs, PGHs, PGIs and PGJs according to the structure of the five-membered ring moiety, and further classified into the following three types by the number and position of the unsaturated bond at the carbon chain moiety:
Subscript 1: 13,14-unsaturated-15-OH
Subscript 2: 5,6- and 13,14-diunsaturated-15-OH
Subscript 3: 5,6-, 13,14-, and 17,18-triunsaturated-15-OH.
Further, the PGFs are classified, according to the configuration of the hydroxyl group at the 9-position, into α type (the hydroxyl group is of an α-configuration) and β type (the hydroxyl group is of a β-configuration).
PGE1 and PGE2 and PGE3 are known to have vasodilation, hypotension, gastric secretion decreasing, intestinal tract movement enhancement, uterine contraction, diuretic, bronchodilation and anti ulcer activities. PGF1α, PGF2α and PGF3α have been known to have hypertension, vasoconstriction, intestinal tract movement enhancement, uterine contraction, lutein body atrophy and bronchoconstriction activities.
Some 15-keto (i.e., having oxo at the 15-position instead of hydroxy)-PGs and 13,14-dihydro (i.e., having single bond between the 13 and 14-position)-15-keto-PGs are known as the substances naturally produced by the action of enzymes during the metabolism of primary PGs.
U.S. Pat. No. 7,064,148 to Ueno et al. (the reference is herein incorporated by reference) describes that specific prostaglandin compounds including bicyclic tautomer of 15-keto-prostaglandin compounds open and activate chloride channels, especially ClC channels, more especially ClC-2 channel.
Recent findings suggest that PGs stimulate bicarbonate secretion in stomach and duodenum by acting on the PG receptors (J Physiol. Pharmacol. 50(2), 155-167, 1999, the reference is herein incorporated by reference).
On the other hand, it is reported that unoprostone isopropyl, one of the 15-keto-prostaglandin compounds does not have affinity for PG receptors such as EP and FP receptors (Journal of Ocular Pharmacology and Therapeutics 17(5), 433-441, 2001, the reference is herein incorporated by reference).